In a carriage printer, such as an inkjet carriage printer, a printhead is mounted in a carriage that is moved back and forth across the region of printing. To print an image on a sheet of paper or other recording medium (sometimes generically referred to as paper herein), the recording medium is advanced a given distance along a recording medium advance direction and then stopped. While the recording medium is stopped and supported on a platen, the printhead carriage is moved in a direction that is substantially perpendicular to the recording medium advance direction as marks are controllably made by marking elements on the recording medium—for example by ejecting drops from an inkjet printhead. After the carriage has printed a swath of the image while traversing the recording medium, the recording medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
In order to produce high quality images, it is helpful to provide information to the printer controller electronics regarding the printing side of the recording medium and the characteristics of the marks printed on the recording medium by the printhead. Information about the recording medium itself can include whether it is a glossy or matte-finish paper. Information about the marks printed on the recording medium can include relative alignment between marks of different colors, angular misorientation of the printhead relative to the direction of relative motion of the recording medium, or relative alignment of marks between left to right and right to left passes in a carriage printer, or missing marks corresponding to defective portions of the printhead, such as bad nozzles in an inkjet printhead. Using the information from the optical sensor, the printer controller is designed to control the printing process to optimize printing quality by using appropriate print modes for the detected media type, by correcting for various types of misalignments and by compensating for defective portions of the printhead.
It is known in the prior art to attach an optical sensor assembly to the printhead carriage of a carriage printer. See for example U.S. Pat. Nos. 5,170,047, 5,905,512, 5,975,674, 6,036,298, 6,172,690, 6,322,192, 6,400,099, 6,623,096, 6,764,158 and 6,905,187. Such an optical sensor assembly can be called a carriage sensor. In the same way that the printhead can mark on all regions of the paper by the back and forth motion of the carriage and by the advancing of the recording medium between passes of the carriage, the carriage sensor is able to provide optical measurements, typically of optical reflectance, for all regions of the paper. A carriage sensor assembly typically includes one or more photosensors and one or more light sources, such as LED's, mounted such that the emitted light is reflected off the printing side of the recording medium, and the reflected light is received in the one or more photosensors. Typically an external lens is configured to increase the amount of reflected light that is received by the photosensor. Typically the photosensor signal is amplified and processed to separate the signal from the background noise. LED's and photosensors can be oriented relative to each other such that the photosensor receives specular reflections of light emitted from an LED (i.e. light reflected from the recording medium at the same angle as the incident angle relative to the normal to the nominal plane of the recording medium) or diffuse reflections of light emitted from an LED (i.e. light reflected from the recording medium at a different angle than the angle of incidence). Diffuse light scattering can be due to local roughness in the recording medium or to localized curvature in the medium for example.
Competitive pressures drive the need to provide high quality printing at lower cost. High quality printing can require smaller dot sizes that the printhead marks on the paper. Typical drop size of modern inkjet printers, for example, is on the order of several picoliters or smaller. Because of this, test patterns for alignment or defective jets can provide a weak signal, and yet these tests must be accurate or the printer controller will not make optimized corrections. Lower cost in the printer can require removing cost from the carriage sensor optics and/or electronics. This can make it even more difficult to accurately sense marks on paper or the characteristics of the printing surface of the recording medium. What is needed is a low-cost design for the carriage sensor and its associated electronics that is consistent with the requirements of high quality printing.